Electrical energy storage device containing a tellurium additive

ABSTRACT

The capacity of an electrical energy storage device containing a high surface area carbon cathode, a metallic anode, and a fused salt electrolyte in contact with the electrodes, can be greatly enhanced by the addition of a tellurium compound directly to the electrolyte or to the carbon cathode. On passage of an electric current through the system, the tellurium becomes permanently bonded to the carbon of the cathode thereby forming an &#39;&#39;&#39;&#39;active&#39;&#39;&#39;&#39; tellurium species that manifests itself as a characteristic plateau in the discharge profile of the cell.

United States Patent 91 Rightmire et al.

[ Nov. 6, 1973 ELECTRICAL ENERGY STORAGE DEVICE CONTAINING A TELLURIUMADDITIVE [75] Inventors: Robert A. Rightmire, No rthfield; Joseph E.Metcalfe, 111; James L.

Benak, both of Bedford Heights, all of Ohio [73] Assignee: The StandardOil Company,

Cleveland, Ohio [22] Filed: Sept. 28, 1970 [21] Appl. No.: 75,893

Related US. Application Data [63] Continuation of Scr. No. 808,876,March 20, 1969,

Pat. No. 3,567,516.

[52] US. Cl 136/20, 136/22, 136/76,

136/121,136/122 [51] Int. Cl. Hlm 35/02 [58] Field of Search 136/22,121, 20,

Primary Examiner-Winston A. Douglas Assistant Examiner C. F. LefevourAtt0rneyJohn F. Jones and Sherman J. Kemmer [57] ABSTRACT The capacityof an electrical energy storage device containing a high surface areacarbon cathode, a metallic anode, and a fused salt electrolyte incontact with the electrodes, can be greatly enhanced by the addition ofa tellurium compound directly to the electrolyte or to the carboncathode. On passage of an electric current through the system, thetellurium becomes permanently bonded-to the carbon of the cathodethereby forming an active tellurium species that manifests itself as acharacteristic plateau in the discharge profile [56] References Cited ofthe cell. UNITED STATES PATENTS 2,081,926 6/1937 Gyuris 136/83 R 1Claim, 2 Drawing Figures TELLURIUM ADDITIVE PRESENT (EXAMPLE 111 o NADDlTlVE t PRESENT (EXAMPLE I) LO I 1 1 \7 I I o 10 2o 6O 7O ELECTRICALENERGY STORAGE DEVICE CONTAINING A TELLURIUM ADDITIVE This is acontinuation of our co-pending US. Pat. application Ser. No. 808,876filed Mar. 20, 1969 now US. Pat. No. 3,567,516.

This invention relates to an electrical energy storage device and to atellurium-containing carbon electrode suitable for use in an electricalenergy storage device. More particularly, this invention relates to anelectrical energy storage cell or battery containing a positiveelectrode comprising a high surface area, porous, carbon containing anactive electrochemically formed tellurium species, a negative electrodeand said electrodes being immersed or in contact with a fused alkalimetal halide or alkaline earth metal halide electrolyte.

It has been discovered that a carbon electrode containing the activetellurium species function as a reversible positive electrode with veryhigh energy storage capacity. The energy storage capacity of a highsurface area, porous carbon electrode can be virtually doubled by addingtellurium to the system in the manner described herein. 1

In accordance with this invention, improved cell capacity is readilyobtained by the addition of a tellurium .compound directlyto theelectrolyte or to the carbon electrode during the electrode manufacture.The tellurium species in the electrode becomes bonded to the carbon andis transformed into an active form upon a cyclic charge and discharge ofthe cell. The discharge profile of a cell containing the telluriumadditive exhibits a characteristic reaction plateau at a potential ofabout 2.5 to 3.4 volts while in the absence of tellurium, the dischargeprofile of the cell slopes downwardly.

The nature of the active tellurium species thus formed is not definitelyknown. It is postulated that with the passage of electric currentthrough the system the tellurium halide is formed at theelectrodeelectrolyte interface. During this time there is believed tobea bonding of the tellurium species to the carbon. Upon alternatecharge and discharge of the cell the halide migrates between the cathodeand the electrolyte, while the tellurium remains attached to the carbonin the cathode. It is thus'possible that the active form may be acarbon-tellurium species halide.

The tellurium may be added to the system as any tellurium compound thatis soluble in the electrolyte, stable in the environment of the cell andis compatible with the ions of the system so that metals foreign to thesystem will neither contaminate nor plate out on the surface of themetallic anode. Those compounds suitable for the purpose of additioninclude tellurium metal, tellurous and telluric halides, oxides, andacids the tellurides and the tellurate and tellurite salts of the alkaliand alkaline earth metals. Examples of these compounds'include TeCl TeClTeBr TeBr Tel Tel TeF TeF TeO, TeO TeO K TeO K TeO Na TeO Na TeO Li TeOLi TeO MgTeO MgTeO H TeO H TeO K Te, Na Te, Li T e, MgTe, and the like.The preferred compounds are those containing cations or anions that arealready present in the system. Those particularly suitable are thetellurium halides and the tellurates, tellurites and tellurides oflithium and potassium.

tem is therefor reversible and there is little chance for loss oftellurium by being transformed into inactive, insoluble, tellurium metalduring the operation of the cell. Any tellurium metal that is lost fromthe carbon cathode is reduced at the anode to the negative valencestate, Te. The anion thus formed is soluble in the electrolyte and willagain migrate from the electrolyte to the cathode where it is bonded tothe carbon and can form the active species.

The amount of tellurium required in the system to bring about adiscernible enhancement in energy storage capacity is more dependentupon the design of the cell and the rate of reaction in forming theactive tellurium-carbon complex, than by the rateof diffusion oftellurium into the cathode. Sufficient amounts of tellurium metal shouldbe present in the system to drive the reaction forming the activetellurium species to completion. However, not so great an excess oftellurium should be present so as to cause excessive leakage current.Tellurium may be added to the electrolyte or to the carbon electrode inamounts such that the conditioned cathode contains from 5 to 40 percentby weight, and preferably from 10 to 35 percent by weight, of telluriummetal, based on the weight of carbon.

To insure good electrical structure and full capacity of the electricalenergy storage cell, easily degradable components in the structure areremoved and the electrodes are permeated with electrolyte for maximumoperational efficiency. This is accomplished by preconditioning theelectrical energy storage cell by immersing the carbon-containingcathode assembly and the anode in the electrolyte and alternatelycharging and discharging at a constant predetermined voltage. Thiscycling converts the carbon into an unexpectedly good electron conductoror negative charge holding medium, and causes the electrochemicalassociation of the carbon with certain constituents from the eutecticmelt of the electrolyte.

The formation of the electrochemically produced active tellurium speciesin the carbon cathode may take place concurrently with thepreconditioning treatment of the electrode, by cycling the cell betweenthe voltage limits of about 1.0 to 3.4 volts. The cathode may be exposedto the tellurium-containing electrolyte melt and cycled appropriatelyuntil the carbon picks up the required amount of tellurium from theelectrolyte. The electrode can continue to cycle indefinitely in thetellurium-containing electrolyte or the electrode may be removed andrecycled in fresh electrolyte prior to cell assembly to minimize leakagecurrent that may result from a large carry-over of telluriumcontained'in the electrolyte-filled pores of the porous carbon cathode.

lln accordance with the present invention the carbon comprising thecathode is a highly porous, high surface area, activated carbon in theform of finely divided particulate material. A broad range of carbons issuitable for this purpose. Carbons in accordance with the presentinvention can be prepared from activated petroleum coke, wood char,activated sodium lignosulfonate char, activated bituminous coal,polyvinylidene chloride chars, polyacrylonitrile chars and the like.

A very useful polymeric electrode material can be obtained bypolymerizing a mixture of a vinyl nitrile monomer and a polyalkenylmonomer containing at least two polymerizable alkenyl groups, as morefully described in U.S. Pat. application Ser. No. 525,558 new U.S. Pat.No. 3,476,603.

The active carbon utilized in the preparation of the cathode has asurface area in the range of l2,000 m lg, and preferably in the range of300-1 ,500 m /g, as measured by the Brunauer-Emmett-Teller method. Thesurface area is mainly internal and may be generated by activation. Thepores in the activated carbon must be of sufficient size to permitelectrolyte penetration.

Although some chars, as for example polyvinylidene chloride chars, aresufficiently active without subsequent activation, many of the carbonsrequire further activation by one of numerous methods, some of which arehereinafter discussed, to impart reasonable capacity and conductivity tothe carbon.

The initial stage in the preparation of an active carbon iscarbonization or charring of the raw material, usually conducted in theabsence of air below 600 C. Just about any carbon-containing substancecan be charred. After the source material is charred, the second step isactivation. The method used most extensively to increase the activity ofcarbonized material is controlled oxidation of a charge by suitableoxidizing gases at elevated temperatures. Most of the present commercialprocesses involve steam or carbon dioxide activation between 800C and1,000C, or air oxidation between 300C and 600C. Alternately, gases suchas chlorine, sulfur dioxide and phosphorus may also be used. The timerequired for activation varies from 30 minutes to 24 hours, depending onthe oxidizing conditions and the quality of active carbon desired.Inhibitors or accelerators can be mixed with the carbon to develop theincreased activity. Other activation methods include activation withmetallic chlorides and electrochemical activation. The latter is aprocess whereby capacity of an electrode can be increased byelectrochemical cycling.

Another general method of activation is the dolomite process. Substancessuch as dolomite, sulphates and phosphoric acid are mixed with thecarbon. On activation, the material continuously releases a uniformdistribution of oxidizing gases to the carbon surface.

Some of the activated carbon is made from hard and dense material. Thismaterial is usually carbonized, crushed to size, and activated directlyto give hard and dense granules of carbon. In other cases, it isadvantageous to grind the charcoal, coal, or coke to a powder, form itinto briquettes or pellets with a tar or pitch binder, crush to size,calcine to 500700C, and then activate with steam or flue gas at 850950C.The latter procedure gives particles with a tailor-made structure whichare easier to activate because they possess more entry channels or macropores for the oxidizing gases to enter and the reaction products toleave the center of the particles.

The tellurium may be incorporated into the carbon electrode by addingthe desired tellurium compound to the activated carbon powder during themanufacture of the electrode. By adding the tellurium compound directlyto the carbon prior to the molding step, a shorter preconditioning cycleis required to convert the tellurium to the active" form than isrequired when tellurium is added to the electrolyte.

Tellurium may be introduced into the electrode by mixing a suitabletellurium compound, such as a tellurate or a telluride, with anactivated carbon powder and binder, in the desired proportions, and theelectrode is molded into the proper geometric form and heated to atemperature of about 900C under an inert atmosphere. V V

The anode may comprise any one of several different metals or metalalloys that are stable in the electrolyte melt. For example, the anodemay be composed of a metal such as lithium, sodium, potassium,magnesium, bismuth, or antimony, or alloys of these metals. Lithium isparticularly suitable and alloys of lithium with such metals asaluminum, indium, tin, lead, silver and copper may also be employed.Ternary lithium alloys can likewise be used. Especially preferred is analuminum-lithium electrode which can be produced by preparing an alloyof aluminum and lithium, or, alternatively, by preconditioning orcycling a substantially pure aluminum electrode in an electrolytecontaining lithium ions, during which preconditioning process lithium isdiffused into the aluminum electrode structure. The former is thepreferred embodiment.

The aluminum-lithium alloy of the electrode comprises aluminum inamounts of from about to weight percent, based on total composition, andfrom about 5 to 30 weight percent of lithium, based on totalcomposition. Incidental impurities such as, for example, copper,magnesium, manganese, indium and iron may be present in quantities lessthan 10 weight percent, based on total composition. An aluminum-lithiumelectrode of this range of composition operates at substantiallyconstant voltage and exhibits high storage capabilities.

The aluminum-lithium electrode is capable of storing lithium metal ofthe electrolyte without forming an extensive liquid. Hence, theelectrode remains solid and is capable of diffusing the lithium metal ofthe electrolyte through its structure. It has been found that on chargeof the cell comprising the aluminum-lithium electrode, the electrodestructure expands wherein lithium metal of the electrolyte enters theelectrode structure; on discharge, the lithium metal leaves theelectrode structure. The electrode must, therefore, be able to withstandthe stresses of expansion and contraction. For this reason, thealuminum-lithium metal electrode is preconditioned prior to use.

If the aluminum and lithium of the electrode are combined bypreconditioning a substantially pure aluminum electrode in a fusedalkali halide electrolyte, as aforementioned, the initial cycling mustbe done slowly. This slow preconditioning results in an electrode ofsubstantially high uniform aluminum-lithium distribution porosity andfacilitates the take-up and release of the lithium metal upon thesubsequent fast charge and discharge of a cell containing the electrode.If the initial charge and discharge cycles of the preconditioning arecarried out too rapidly, local regions of liquid metal alloy are builtup, and the result is pitting of the aluminum-lithium electrode whichhas a deleterious effect when the electrodes are put into routine use.Evidence of such pitting is visually evident in the electrodes,indicating lithium agglomeration. Aluminumlithium electrodes cycled byslow charge and discharge show a fine, even distribution of the lithiummetal in the aluminum.

The electrolyte used in the device of this invention is a mediumcomprising a source of dissociated metal and halide ions which aremobile and free to move in the medium. Fused salt mixtures containingalkali metal and alkaline earth metal halides, as for example lithiumchloride, potassium chloride, sodium chloride, calcium chloride, calciumfluoride, magnesium chloride, lithium bromide and potassium bromide, canbe used. The lowest melting point media are most desirable. However, itis contemplated by the present invention that the medium be operable inthe liquid state at temperatures in the range of 350600C.

Typical examples of materials which can be used as binary saltelectrolytes include lithium chloridepotassium chloride, potassiumchloride-magnesium chloride, magnesium chloride-sodium chloride, lithiumbromide-potassium bromide, lithium fluoride-rubidium fluoride, magnesiumchloride-rubidium chloride, lithium chloride-lithium fluoride, lithiumchloridestrontium chloride, cesium chloride-sodium chloride, calciumchloride-lithium chloride, and mixtures thereof.

Examples of ternary electrolytes are calcium chloride-lithiumchloride-potassium chloride, lithium chloride-potassium chloride-sodiumchloride, lithium chloride-potassium chloride-magnesium chloride,calcium chloride-lithium chloride-sodium chloride, and lithiumbromide-sodium bromide-lithium chloride.

. The preferred electrolyte systems are those of potassiumchloride-lithium chloride and lithium bromide and potassium bromide, andmixtures thereof. A lithium chloride-potassium chloride system of 41mole percent potassium chloride and 59 mole percent lithium chlorideforms a eutectic which melts at 352C and has a decomposition voltage ofabout 3.55 volts.

Since the electrical energy storage device operates at or abovethe-fusion temperature of the electrolyte, the above-mentionedelectrolytes are provided a means of heating to insure their remainingin the molten state.

The electrical energy storage units hereindescribed lend themselves toconnection with units of similar construction either by connection of anumber of units in series and parallel, or by utilization of a stack ofelectrodes.

This invention will be further illustrated by reference to the drawingin FIG. 1 wherein a schematic test cell of the present invention isshown. The telluriumcontaining carbon electrode 12 and metallic anode 11are positioned one from another, in spaced relationship, immersed in anelectrolyte 17 held in a heatresistant glass tube or stainless steeltube 18. Carbon electrode 112 is fixed rigidly to a graphite currentcarrier 13 and the metallic anode 11 is fixed rigidly to a steel currentcarrier 14. The container comprising the electrolyte and electrodes ispurged of atmospheric air and an inert gas is introduced into thecontainer. The open end of the container is then sealed with a cap 15 ofinert material, such as lava or ceramic.

A better understanding of the present invention can be obtained from thefollowing examples. The examples are illustrations of specificembodiments of the invention and are not to be construed in any way aslimitations of the invention. The experiments were carried out in thesame type of test tube cell herein aforementioned, and shown in FIG. 1.

Example I A cathode was obtained from a charcoal prepared by Pure CarbonCompany. The commercial-grade carbon had the following physicalproperties: a total pore volume of0.566 cm lg, a surface area of 400 m/g, an average density of 0.90 g/cm, a Scleroscope hardness of 35-45,and an ash content of 10 percent. The carbon electrode contained 0.092inch of active carbon.

The solid aluminum-lithium anode, approximately 1.0 X 0.5 X 0.15 inches,initially contained 13 percent by weight of lithium. Both electrodeswere immersed in an electrolyte containing grams of a eutectic saltmixture of lithium chloride and potassium chloride. The eutectic mixturehad a composition of 59 mole percent lithium chloride and 41 molepercent potassium chloride and had a melting point of 352C. The cellassembly was contained in a stainless steel test tube with a totalinside volume of 13.4 inches An argon atmosphere was established withinthe cell and the cell was operated at a temperature between 450C and500C. The electrodes were conditioned in the electrolyteby cycling thecell at a constant voltage of 3.34 volts for 30 minutes and thendischarging at a constant current to 1.0 volt. The cell was cycled for20 cycles. At a constant current discharge of 200 milliamperes the celldelivered watt minutes per inch of carbon.

Example 2 The experimental conditions of Example 1 were repeated exceptthat 1.6 grams of potassium telluride (K Te) were added to theelectrolyte in the discharge state and the cell was then cycled in thesame manner as described. At a constant discharge of 200 milliamperes,the cell delivered 315 watt minutes per inch of carbon.

A comparison of the discharge curves obtained for the cells of Examples1 and 2 is shown in FIG. 2. A sloping discharge profile is observed forthe cell in Example 1 without the tellurium additive, whereas with thetellurium additive, in Example 2, a characteristic plateau in thedischarge curve appears at a reaction potential of about 2.5 to 3.34volts. As a result of a shift in the reaction to a higher voltage, asignificant increase in watt minutes is observed for the cell containingthe tellurium additive.

Example 3 The cell and operating conditions of Example 2 were repeatedexcept that 1.5 grams of potassium telluride (K Te) were added to theelectrolyte and the carbon electrode had a volume of 0.124 inch. At aconstant discharge of 400 milliamperes, the cell delivered 416 wattminutes per inch".

Example 4 lithium anode of the auxiliary cell had the dimensions livered297 watt minutes per inch of carbon cathode.

Example 5 The operating conditions of Example 2 were repeated exceptthat the molded cathode was obtained from Pittsburgh Activated CarbonCompany. The density of the material as determined by Hg displacementwas 0.92 g/cm and the electrode contained 0.101 inch of active carbon.1.5 grams of potassium telluride were added to the electrolyte. At aconstant current discharge of 400 milliamperes, the cell delivered 502watt minutes per inch of carbon cathode.

Example 6 A carbon cathode was prepared from a saran polymer resin(polyvinylidene chloride) obtained from Dow Chemical Company. The saranwas charred at temperatures of up to 900-l ,000C in an argon atmosphereduring a 16-hour heat-treatment cycle. The char was ground and sievedthrough 100/325 U. S. standard mesh screens. 30 grams of the char wereblended with 12.85 grams of a phenol-formaldehyde thermosetting resin ina twin shell blender. The mixture was molded in a die at 6,000 psi andsubsequently baked to 900-l ,000C in an argon atmosphere during a16-hour heat-treatment cycle. The electrode contained 0.0625 inch ofactive carbon and had a surface area (B.E.T. method) in the range of500-1,000 m lg, an ash content of a 0.2 percent, and a l-lg density of0.97 g/cc.

A cell containing the above prepared carbon cathode, a solidaluminum-lithium anode, approximately 1.75 X 0.50 X 0.15 inches, andapproximately 160 grams of LiCl-KCl eutectic electrolyte was assembledin a stainless steel test tube. The saran cathode cell produced 288 wattminutes per inch of carbon. To the cell having an open circuit potentialof 1.0 volt, grams of potassium tellurate (K TeO were added to theelectrolyte. The cell was cycled at a constant voltage of 3.34 volts for20 minutes and discharged at a constant current of 300 milliamperes froma cell voltage of 3.34

to 1.0. The capacity of the cell increased over a period of 30 days to amaximum of 617 watt minutes per inch and then leveled off.

Example 7 The experimental conditions of Example 1 were repeated withthe exception that the carbon cathode utilized was prepared as follows:13 grams of a thermosetting, phenol-formaldehyde resin binder, 11 gramsof 1( TeO and 30 grams of a coal-derived carbon char having the physicalproperties:

Surface area 950-1050 m /g Hg density 0.75 g/cc Helium density 2.1 g/ccPercent ash 8.0 percent Pore volume 0.85 cc/g were blended and moldedunder pressure. The molded electrode was heated to 900C in an argonatmosphere for 16 hours. The electrode was placed in a cell as describedin Example 1. The cell was charged at a constant voltage to 3.34 voltsfor 35 minutes. The discharge profile of the cell exhibited thecharacteristic tellurium complex plateau and the cell produced 540 wattminutes per inch of carbon. The conditioned cathode had a capacity of414 watt minutes per inch of carbon.

We claim:

1. An electrode consisting essentially of carbon and tellurium preparedby the process of placing a carbon electrode in an electrochemical cellcontaining a negative electrode and a fused halide salt electrolyte towhich has been added a compound of tellurium which is at least partiallysoluble in the electrolyte, said carbon and negative electrodes being incontact with the tellurium-containing electrolyte, and cycling saidelectrochemical cell alternately in a charge and discharge direction toeffect the electrochemical bonding of tellurium to the carbon in theelectrode and thereby electrochemically forming a tellurium carboncomplex.

